text:
What's the difference between a balked landing and a go-around?
Is there a difference between a balked landing and a go-around?
If there is, what exactly is a balked landing?

The only term in the Pilot Controller Glossary is Go Around, so should technically be the only term used, at least when communicating with ATC:

GO AROUND- Instructions for a pilot to abandon his/her approach to landing. Additional instructions may follow. Unless otherwise advised
  by ATC, a VFR aircraft or an aircraft conducting visual approach
  should overfly the runway while climbing to traffic pattern altitude
  and enter the traffic pattern via the crosswind leg. A pilot on an IFR
  flight plan making an instrument approach should execute the published
  missed approach procedure or proceed as instructed by ATC; e.g., "Go
  around" (additional instructions if required).
(See LOW APPROACH.)
(See MISSED APPROACH.)

The ATP PTS also includes a Rejected Landing, which is what our simulator instructors refer to as a Balked Landing, and must be at or below 50 feet AGL.  It has the following note describing it in the PTS:

NOTE: The maneuver may be combined with instrument, circling,  or missed approach procedures, but instrument conditions  need not be
  simulated below 100 feet above the runway.  This maneuver should be
  initiated approximately 50 feet  above the runway or landing area and
  approximately over  the runway threshold or as recommended by the FSB
  Report.

The pilot follows the same procedures for both of them by aborting the landing, cleaning up the airplane, and flying the appropriate procedure afterwards.

I'm thinking "balked landing" might be some kind of local terminology.  I've never heard that one before, it's always "go around".

@BrianKnoblauch Yes, as I said that's the "official" term that should be used by ATC.  I've only ever heard balked landing in the training environment.

Yeah even if you bounce it off the pavement it is still a go-around. It might be considered a balked landing below DH but over the radio it's "N1234 going around.." or going missed I guess if your doing an instrument approach.

also note that the little button on the autopilot is the "go around" button, not the "balked landing" button: http://en.wikipedia.org/wiki/Takeoff/Go-around_switch

Why the "instrument conditions need not be simulated below 100 feet above the runway" thing in the rejected-landing note?

CFR 23.2120 and 23.2130 refers to 'balked landing' regarding climb performance, stall speed margin and minimum control speed. 'go around' is not used anywhere in Part 23.

Almost everything I read says that a balked landing is the same thing as a go-around. (Indeed, the wikipedia page for balked landing redirects to the 'go-around' page).

In these cases, a balked landing, also known as a go-around, creates a better opportunity for a near perfect landing [src]
A balked landing – also known as a go-around, is an aborted landing of an aircraft that is on final approach … for landing [src]

This thread mentions that they are the same as a go-around, but can include when the wheels briefly touch down.
However, I did find one slightly dissenting opinion here:

I don’t know of an official definition of a balked landing that makes it different from a go-around. The way the balked landing term is generally used is that the actual landing procedure has begun and must be aborted. A go-around generally begins at a higher altitude and lacks the urgency of the balked landing.

So, in summary, they're pretty much the same thing (or exactly the same thing), but some people differentiate the two by the altitude the procedure begins at.

I'm accustomed to the terms meaning the same thing too. I suppose from a purely grammatical/linguistic standpoint "to balk" means "to hesitate or be unwilling to accept an idea or undertaking", so when one "balks" a landing (unwilling to land because the approach is bad, the winds just shifted, the ILS died, a truck pulled onto the runway, etc...) the result is a go-around - hopefully a successful one.

@voretaq7 I agree with you, you should put that in an answer. The go around is a procedure (and in my opinion extends beyond the balked landing), balking the landing is just that, not landing. It has (as you say, hopefully) the same result, but they have, as I understand it, "linguistically" different scope.

@roe I consider it a minor grammatical nitpick -- in aviation the two terms are essentially interchangeable in my experience (though "Go Around" is the only officially defined term)

While there is no distinction between balked landing <-> go arround, "technically" you will find in regulations a difference between balked landing and missed approach.
Missed Approach climb is defined as a go-around from at or above DH (possibly with one engine inoperative). CS(Part) 25 assumes required gradient on:

Go-around thrust on remaining engines
Landing gear retracted
Approach flap set

Balked Landing climb is a go-around from below DH, possibly in the flare (note that all engines are assumed available). CS (Part) 25 assumes required gradient on :

Go-around thrust all engines
Landing gear down
Landing flap set

That’s my understanding of the difference. The consequence of the above is that for normal operations, there is no real difference. However, go-around with an engine failure is different from a balked landing with an engine failure in so far as regulations require the operator to only consider the first case when calculating terrain and obstacle clearance. This means that, from a purely legal standpoint, a balked landing with an engine failure is not a manoeuvre where safety of continued flight is assured, while a go-around with an engine failure is.

Balked Landing is a very low altitude Go-Around. Not a term you would use with ATC, they don't care. It's a term found in certain aircraft flight manuals and requires a slightly different procedure than a normal go-around, similar to a rejected landing. Usually wheel touchdown is likely and you must verify airspeed increasing past a certain point (safe flap retract speed) and positive rate of climb before de-configuring, otherwise you risk touching down with the gear in transit or slamming down because of loss of lift from flaps/slats.

Can you provide links to, or at least descriptions of, the "certain aircraft flight manuals"? It would be interesting to know which aircraft require these different procedures.

The missed approach procedure is published with considering the terrain in the area. It is calculated that an aircraft can safely go around clearing the terrain if it starts go-around at the DH. If the aircraft descents below DH, and decides to go around things go different. Because now it starts the procedure approximately 200' below and also 1NM ahead of the published missed approach procedure. This is a balked landing, and has to be handled with more care. to carry out the engine out SID after balked landing in low visibility conditions also has to be considered

A balked landing is a go around in the landing configuration: flaps and slats at the required setting, gear down, etc.
A go around can be performed in any aircraft configuration.

Balked Landing – A discontinued landing attempt. Term is often used in conjunction with aircraft configuration or performance assessment, as in “Balked landing climb gradient”. Also referred to as  “Go-Around”

balked landing :  is a go-around initiated after touchdown but before thrust reverser selection .
(source: Term from Boeing bulletin)

Term from Boeing bulletin.

A Balked landing is not a simple go around or missed approach landing. In a balked landing the airplane must be able to maintain a steady gradient of climb of at least (different for each airplane) with:

(1) Not more than the power that is available on each engine eight
seconds after initiation of movement of the power controls from
minimum flight-idle position;
(2) The landing gear extended;
(3) The wing flaps in the landing position; and
(4) A climb speed equal to VREF

Source: http://www.risingup.com/fars/info/part23-77-FAR.shtml

A baulked landing and a go around are very much TWO different procedures. A baulked landing only happens on very short final if for instance the captain isn’t happy with what they see. The procedure is simply to pitch nose up and simultaneously apply full power. The wheels may touch the runway but that is fine. This is an escape maneuver to quickly get away from the ground. Once the aircraft is in a stabilized climb, the go around procedure can be applied. This is initiated by a term like “Go around, flaps” which helps bring both pilots back into the loop.
A go around procedure always follows a baulked landing, but a go around procedure can of course be flown on its own.


text:
Why is the wing bending so much in this video?
On a recent vlog, YouTuber CGP Grey mentioned how much the wing of the plane he was on bent. I understand that wings are designed to bend a little,  but what actually causes so much deformation?
Here it's a video of it:

Why does this happen?

Why does this happen? Because its designed to.

@RonBeyer I get that it can, what physically makes it bend?

@Pureferret, the weight of the aircraft it is carrying.

The wing is quite literally holding the entire weight of the aircraft. Almost every kilogram is being held in the air by those wings!

I've no idea what the superimposed blue line is. It's certainly not the position of the wing when unloaded.

This isn't a duplicate. The other question is asking what aspects of the design allow it to bend; this question is what causes the actual bending.

@ Ron Beyer how is your comment helpful?

This view of the wing is considerably foreshortened, which exaggerates the apparent flex. The flex seen in sailplanes is much more dramatic, and, I would guess, also more so in a 787 or A350 - though not to sailplane levels - on account of its composite wing (some hints in the video suggest this was a 747.)

Grey has seriously disappointed me with this video and the remarks about wing flexing, I was expecting him to be more rational.

@dan I figured it would be a rigid design, and for mthe video, that the bending Grey noticed was unusual.

@Pureferret define "unusual". saying "I've never seen it before" does not make it unusual.

@Federico unusual, as in not usual. That is to say Grey implied compared to what he usually saw, the wing bending so much was 'not usual'.

@Pureferret that's my point. the fact that one person does not usually see it, does not make the thing not usual.

@sdenham I was about to ask what plane it was.

Because that wing is a cantilevered beam which is now supporting over half the weight of the jet on it.  This causes the wing to flex, much like a leaf spring, when loaded.  While fascinating to watch this is benign and designed to do this.  In addition, this wing flexure helps to absorb bumps, air turbulence, dampen flutter, maneuvering loadings, etc. much like the coil springs in your car's suspension do.  Don't be alarmed to see this.  Those wing structures have been tested to far worse loads and conditions that you will ever see on a revenue carrying flight before failure occurs.  A 747's wingtips can flex by as much as 12ft up/down during flight.  Newer composite wings are capable of much greater flexure.
This was video shot of Boeing's destructive testing on the 777 wing.  Note the wingtip flexes by as much as 20ft upward at a loading over 150% that of its design load limits before structural failure occurred.

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text:
Would flying below FL285 be feasible for aircraft that don't comply with EASA CPDLC?
I am not a pilot, my understanding about flight is limited.
EASA CPDLC will require airplanes to be capable of PM-CPDLC data communication to fly above FL285.
For an airline with multiple 767-300 and 757-200 airplanes, would it be feasible to consider the option of flying below FL285 in European airspace, instead of ceasing operation in the region or investing millions of dollars for avionics upgrades?

Unlikely. The extra fuel burned would cost a lot more than the required Avionics upgrade.
Both the 75/767 would be well below optimum cruise level.

Generally true, but I would say that it depends on the route.  Flying from the US to London would probably be okay because you would be descending anyway.  Flying to (or within) Eastern Europe would be another matter though....

The adoption of CPDLC is more or less voluntary, because they have created "exceptions" for practically every single aircraft that does not have it built in. Flying below 28K is not practical except for short flights, but it does not matter because it unlikely they will start banning aircraft that do not have the equipment. Note that there are international treaties in place that mandate countries allow commercial overflight and if you just started banning various aircraft, you would be in violation of those treaties.

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text:
Would a 767-300 be considered FANS-1 capable without SATCOM?
It is known that aircraft flying in Europe above FL285 will be mandated to be capable of PM-CPDLC data link communication. Our 767-300 has ACARS CMU, Pegasus FMC, MCDU and VHF radios (three). The ACARS system works fine, but there is no SATCOM.
As indicated on COMMISSION REGULATION (EC) No 29/2009, aircraft with an individual airworthiness certificate issued before 1 January 2014 that are equipped with Future Air Navigation System (FANS-1) are exempt from the requirement of the PM-CPDLC for the lifetime of the aircraft.
My question is, would our 767-300 aircraft with the above avionics be considered FANS-1 capable and qualify for the exemption? Any suggestions on how I can confirm that?

You would ask the maker of your MCDU whether it's FANS-1 capable using just a VHF datalink. Satcom isn't necessarily a requirement, but you would only be able to use it where VDL is provided, for instance in Europe but not on oceanic routes.


text:
How do we calculate top of descent (TOD) in flight?
What is the in-flight trick?
I know how to calculate it in theory, but how can I calculate it fast? For example, I know I can use this formula for calculating the rate of descent:
[ground speed + add 0]\2
I'm trying to find a formula like that for TOD.

Any approximate method is going to vary for different types of plane, depending on their speed, altitude and descent rate, but here's how I figure it for my Cessna 172.
I know I'm traveling around 2NM a minute (actually a little slower), and I like to descend around 500 fpm.
So if I have to drop 5,000 feet to pattern altitude, the TOD is 10 minutes out (5,000 feet / 500 fpm).   At 2NM a minute, that is 20 miles out.
So if I have my GPS / DME set to my destination airport, I am looking for both ETE: 10 min, and also Dist: 20 miles, then I start my descent.
Its approximate, but its pretty close for planning purposes.
Its always just:
[Altitude Drop] / 500 = Minutes
Minutes * 2 = Miles

what is your Resource for this formula?

Altitude drop mean decent rate?

could you explain the formula?

No, altitude drop and descent rate are not the same thing.  If I'm at 6,500 feet, and I need to descend to 1,500 feet to enter the pattern, then I need to go down 5,000 feet.  That is an altitude drop of 5,000 feet.    Descent rate is is the Feet-Per-Minute rate at which you lose altitude.

The formula comes from basic math and physics.  This should be covered by high school.  See http://www.math.com/school/subject1/lessons/S1U2L3DP.html

Wow , thank you very very much . Your the best

I don't know it's my correct answer yet.  i have to ask my flight teacher

We always taught the rule-of-thumb:    3 x The altitude in miles from landing and descent by 5 x airspeed in FPM. So if you are at 10,000' and 100 knots, you would start your descent 30 miles out at 500 FPM rate of descent.

On a large aircraft, an often cited trick is to use 3 x altitude to airfield, divide it by 1000, and then that is still air distance from airfield to top of descent in nautical miles. For example, from 30000ft, TOD would be 90NM out. This works to roughly achieve an idle descent (if the aircraft’s glide ratio is around 18 - which, give or take, it is for modern airliners in clean configuration at moderate speeds).

This rule of thumb is based on a 3° descent path.

@JWalters Yes, same difference - it works in clean configuration from cruise to approach, and then again down the glide path in approach/landing configuration.

You can refine that by considering if you're approaching from a downwind, base, or straight-in direction. Also, one can refine it further by considering the distance required to slow from descent speed (between 280-320 knots, typically)  to 250 knots before descending below 10,000', and then slowing from 250 knots to approach speed. The "close-as-duplicate" question/answers go into these computations. But 3x is a pretty good general ballpark.

So you say this formula based on 3dgree ?

It’s the same - 3° translates into a descent gradient of 0.052 which translates (for idle thrust) into a glide ratio of roughly 19. The three are the same physical relationship, so it can be based on either. In flying context, I choose to think of the enroute part based on glide ratio (as I‘m ideally descending with idle thrust, accepting as descent angle whatever the aircraft achieves) and the final descent based on 3° (as I‘m following a fixed glide path with thrust set to match whatever required). For modern jets, the values are roughly the same, but they are based on different assumptions.


text:
Why do ATC displays use flight data sent by the aircraft they are tracking?
Why does ATC use data coming from an aircraft itself instead of using the readings from the airport radar?
Also could Aeroperú 603 have been saved if the pilots had ignored the pitot static altitude and instead listened to the "Altitude Too Low" warning that came directly from the Radar Altitude system (i.e. one that works by using radio waves to determine the altitude)?

You seem to have two different questions here: why does ATC rely on data transmitted by aircraft rather than on data from ground-based radar; and could the crew have used their onboard radar altimeter as a substitute for their pitot-static system? Please try to ask one question at a time, as far as possible, it helps to get better answers.

The altitude data on ATC screens comes from the static system of the aircraft, because that is the only sufficiently precise source. The area primary radar is, at long range, only accurate to maybe a mile and that applies to vertical position too. For horizontal position when the separation is 5 nautical miles this is good enough. But the vertical separation is 1,000 ft and the radar simply can't distinguish that. So the aircraft transmit their altitude in the Mode C and Mode S (ADS-B) and that is used for altitude display.
Also, the on-board instruments measure barometric altitude and that is what aircraft are instructed to fly. But a precise radar can only measure geometric altitude, which due to changing atmospheric condition is different, so it wouldn't help anyway.
In future, the position data will more and more be also obtained from the aircraft. The ADS-B broadcasts GPS position and this is more precise than the primary radar and is available even over open sea where primary radar coverage is not possible. Therefore newer systems depend on what the aircraft itself reports more, not less.


text:
Was there ever a fuel-injected Spitfire?
I couldn't find one but did it ever exist? Would adding fuel injection have improved combat effectiveness or fuel efficiency?

What do you mean "how would it work"? Why would it work different than any other fuel injected motor? These hypothetical questions are difficult to answer.

I couldn't find one but did it ever exist?

I do not think so (at least not off the assembly line) the RR Merlin Engines were Carb'ed. The Griffon powered planes were also carb'ed however they used an injection carb which is a bit different than a float style carb but not full on fuel injection.

Would adding fuel injection have improved combat effectiveness or fuel
  efficiency?

Yes, it would have allowed sustained G maneuvers which are not possible in a carbureted plane as the float would fall causing fuel to enter the carb unrestricted.
As per wikipedia...

However, the Merlin's float controlled carburettor meant that both
  Spitfires and Hurricanes were unable to pitch nose down into a steep
  dive.

This was later remedied by installing a small plate over the float to prevent starvation.

...original production variants of the Merlin used an SU manufactured
  carburettor in which the fuel flow was metered through a float. In
  most circumstances this proved to be sufficient but during the air
  battles over Dunkirk and during the Battle of Britain it was found
  that whenever the Merlin was subjected to negative "g" forces, such as
  a quick "bunt" into a dive, the engine would briefly lose power
  through petrol starvation...
...The remedy, invented by Beatrice "Tilly" Shilling, was to fit a metal
  diaphragm with a hole in it, across the float chambers. It partly
  cured the problem of fuel starvation in a dive. The device was
  commonly referred to as 'Miss Shilling's Orifice'....

And then .... the Bendix-Stromberg Pressure Carburettor, circa 1940.

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text:
Why does it take so much fuel to start a jet engine?
I just watched an Abrams tank documentary and it said something like it takes "1 liter of fuel for every mile but 39 liters for a startup." Is it true of turbine engines in general that they need a lot of fuel to start? If so, why?

I heard the same about our tanks, though those had piston engines. I always felt those numbers are heavily exaggerated, because 1: there would be hardly any place left for air in the motor an 2: this amount if fuel needs 350m^3 of air to burn completely. Or: given the excess of air on a diesel/jet engine, that would create a storm around the tank. I'd first check if  the number is true. I can imagine this is the amount a dry tank sucks in to flush all its fuel lines.

A common urban myth is that X takes so much energy to start up, you should just leave it running.  Fluorescent lights burn 10 minutes of power to start, computers use 30 minutes of energy to boot up, it takes more energy to rechill your house than to run the A/C all day, yada yada.   They are all not only false, but thermodynamically impossible.

@Harper for some devices there is some truth in them, a combustion  engine need to get up to speed to be able to idle, an electric motor will suck the most current when stalled/stopped, a fluorescent needs to heat up the electrodes and charge an inductor before it can try and initiate a spark to light it, a computer will exercise all its components on startup, etc. Granted the extra energy needed is on the order of seconds (if that) rather than minutes/hours.

The first figure, 1 litre per mile, is very wrong. The second, 39 to start, is most probably the fuel required for a warm up from a cold start. The Abrams tank weighs 62 tonnes, so you won't get 1 mile on a litre. Its fuel consumption is quoted as 0.6 mpg, or 1.66 gallons per mile, which is 6.3 litres per mile. Quite a difference.
The engine is the Honeywell (previously Lycoming) AGT 1500. It has a two spool gas generator, with a two stage power turbine. It features a heat exchanger through which the HPC exit air flows, to preheat it from the exhaust gas, prior to combustion, to improve engine efficiency.
The engine has a tactical idle that keeps it running at 1500 rpm instead of 950 so that it can rapidly accelerate from a stationary position if needed to avoid or engage a hostile force. In tactical idle it consumes 30+ gallons per hour, instead of 10 at the non tactical idle setting.
It's stated that "in 1990, Project on Government Oversight in a report criticized the M1's high costs and low fuel efficiency in comparison with other tanks of similar power and effectiveness such as the Leopard 2. The report was based on data from U.S. Army sources and the Congressional record."
Some critics say the tank fuel consumption is so poor, that in combat, it has to often wait for the fuel supply logistics to catch up. Other reports deny this. (See paragraph at the very top of the page).
The M1 Abrams was chosen after a competition in June 1973, between the Chrysler Corporation and the Detroit Diesel Allison Division of the General Motors Corporation to build prototypes of a new tank designated M1, and later named the Abrams tank. In November 1976, it was announced Chrysler won the competition.

The M1 was faster and more maneuverable than its predecessors in the M60 series, while offering a lower, smaller silhouette. Besides the substantial gains in performance, the Textron Lycoming AGT-1500 turbine engine was far more reliable than the diesel tank engines then in use with the Army. There was still another benefit. The engine change, despite a penalty in fuel consumption, also resulted in much quieter operation, so much so that soldiers encountering the tank in early maneuvers started calling it "Whispering Death".

The Abrams wikipedia page also states a figure of 6.3 liters per mile, and also states the value of 38 liters to start. As suggested by user3528438 (thank you), the 39 litres (10 US gallons) is quite probably the fuel required to complete a warm up from a cold start. At the tactical idle setting, this would be the fuel consumed in about 15 to 20 minutes.

If it's a cold start it might take minutes to warm up. If a tactical idle is 30 gallons per hour,  39 liters for a warm up run isn't too bad.

Okay. A now about the jet engine...

@user3528438. Yes, thinking about it since writing the answer, I think that's what the 39 litres is required for - not starting, but the warm up. I will modify the answer to include your comment.

@Peter. The engine being discussed is a gas turbine, the Lycoming (now Honeywell) AGT 1500. I will modify the answer to make that clear.

Outrunning the supply chain is a compliment, not a criticism.  This has been a problem since WWII and reflects the good performance of the tanks.  And it's more a case of being able to secure the supply chain.  The precious tankers could follow close behind the tanks, but would be cut to pieces by uncleared enemy units who hid from the tanks, or even local irregulars, farmers with guns etc.

@Harper. Yes, you're right. But I probably didn't express it very well, it's more an issue of a lack of range, than too much speed. Regards


text:
Doppler shift in appendix G of ATSB report for the MH370 incident
Two people provided answers to my previous questions:

What is the nominal frequency of the satellite communications terminal on MH370?
How does one interpret the satellite data on flight MH370?

And they referred me to Appendix G. "Explanatory Notes on BTO and BFO Analysis".
Now I have questions about Appendix G.

The report (Appendix G) says that the Doppler shift from the satellite (down-link) to GES depends on the satellite movement with respect to GES, since the satellite is not exactly stationary.  But in Table 6. the Down-link Doppler frequency varies, from the satellite to GES depending on the aircraft Latitude. Why should the Aircraft Latitude affect the DeltaFdown Doppler frequency shift?
And another question is about the Down-link doppler-shifts, from the Satellite to GES, given in Table 5. The listed Doppler-shift in frequency is -75.1 Hz for UTC time 17:07. However, I can only derive this frequency using the satellite velocity for UTC time 16:55:00?  The calculated frequency differs slightly from the one in the report.

While I share your interest in this subject, I worry these questions are getting to the point where they don't have a lot to do with normal aviation. More attention to spelling MH370 etc correctly, providing links to info you mention and formatting would probably go down well with the audience here. Just my 0.02 worth. I'll fix that for you on this question.

Your reply does not answer the question I asked.  You include the aircraft velocity and heading as viewed from the satellite's location as a possible explaination.  But the DeltaFdown is suppose to only apply to the satellite movement wiith respect to GES.  The Aircraft velocity should not affect the the DeltaFdown value.  The same goes for the Aircraft Latitude. I now understand that Mr. Duncan Steel asked the same question about the aircraft Latitude several days ago.  Getting answers as to how the Log- Data was derived may help in locating Flt370.  Reasonable questions should be encouraged.

For 16:55 UTC, I calculate a doppler frequency of -74.5 Hz for DelFdown.  For 17:05 UTC, I calculate a Doppler frequency of -71.2 Hz for DelFdown. These calculations are based upon the relative movement of the satellite with respect to GES.  If there are other variables that contribute to DelFdown then the report should state so.

Being reasonable is a necessary but insufficient condition for questions here. Questions should also be on-topic and it appears that many people feel this question does not meet that condition. 2) It is better that comments on answers are attached to the answer not to the question. I'd delete my answer but it contains some links and info that may be useful to future readers.

1. Why should the Aircraft Latitude affect the Doppler frequency shift?

It seems to me that table 6 is a check to test whether the equation on page 55 can reliably be used to predict the possible locations of an aircraft from a measured BFO. It does this by using the equation to predict what the BFO should be for a given set of inputs and comparing that with the measured BFO. To see how reliable this is they vary one of the parameters, latitude in this example, to show that the predicted BFO is considerably different for different latitudes and that the best match of predicted BFO to the measured BFO is for the known location of the aircraft. Therefore you know that the BFO is useful - it can be used to rule out some locations.
If you hold constant all the other factors and vary latitude why should that affect BFO?
I guess it's because for the same aircraft heading (with respect to true north), different latitudes but same headings mean different angles of flight with respect to the satellite. That means that there are different magnitudes of component of velocity that is parallel to the line from satellite to aircraft - those different magnitudes represent different speeds in a line directly away-from or directly towards the satellite. This affects doppler.

Note: The above doesn't explain why ΔFdown varies with aircraft latitude.

Other independent analyses of this data

Dr Richard E Cole, University College London
Dr Duncan Steele (who asks the same question)

I'm pretty sure Duncan Steele is smarter than me, so you may be on to something.

2. However, I can only derive this exact frequency using the satellite velocity for UTC time 16:55:00?

That's more of a statement than a question. I guess your question is:

why do my calculations give a different answer?

I see two possibilities

you made a mistake.
the ATSB made a mistake†.

In order for readers to help you with the first or confirm the second, you'd really have to share your calculations with us.

† And presumably Inmarsat, the NTSB, the AAIB and others.

And for checking any calculations, there is probably more people who can help with math over at http://physics.stackexchange.com/.

I withddraw my comments about a possible error in Table 5 of Appendix G regarding the -75.1 Hz. DelFdown doppler shift.


text:
What is the maximum safe bank angle of a 747?
This question arose during the comments of another question, linked for the curious:
What does it take to turn a 747 around 180 degrees?
Because the discussion was interesting, (and to avoid congesting the comments of another user's question), I reposted it here.
Does anyone know the answer to this? Official sources/experience are preferred, but I'll take what I can get.

If I was at altitude and suddenly realized we were about to cross into airspace that might get us shot down, I would, smoothly, do 3 things simultaneously: (1) start the nose down to around 25 degrees down, (2) start a roll to a bank angle of 60-80 degrees, and (3) start the power back to idle. At the same time, I'd ask the first officer to deploy the speed brakes, which would give me great roll response. –  Terry

To which I replied:

80 degrees of bank would be a bit excessive no? 60 might be reasonable for an emergency turn, but certainly no more in a 747? (G-forces aside) –  HCBPshenanigans

Terry's reply:

I was thinking of a range because, in an emergency turn, hitting an
  exact degree of bank would be hard. I would, however, not want to go
  beyond 90 degrees because of the possible disorienting nature for the
  flight crew. At 60 degrees, approximately 87% of the wings lift would
  be turning the airplane. At 80 degrees, about 98% of the lift would be
  turning the airplane, so the lesser bank would sacrifice 11% of the
  turning force. I would expect to lose a lot altitude in the turn. I
  can't think of a reason not use 80 degrees. Pax comfort would be a
  non-issue. –  Terry

My most recent response:

Idk if you are a real 747 captain (there are quite a few on here), but
  iirc many 747 crashes have happened because when the aircraft banks so
  hard that it loses that much vertical lift, it is very difficult to
  recover from. In addition, some flight systems might malfunction under
  these rather extreme conditions. 60, hell even 70, is much steeper
  than it seems. I seem to remember seeing an episode of that old Air Disaster show about just this. –  HCBPshenanigans

Edit: What kind of difference would the difference between 60 and 70 or 80 degrees of roll make in time to turn 360 degrees?

I'm not up on current abbreviations, what does ldk and iirc stand for?

@Terry 'I don't know' and 'If I remember correctly'

Idk if you are a real 747 captain : He is a real 747 captain, See the resume on his linked website: 18 years of flying — typed CE-500, SA-227, B-727, B-747 — last 10 years primarily international on the 747.

Load Factor and Maneuvering Limitations suggests maximum bank angle of 66.5° for a 747-400 with maximum load-factor of 2.5.

@RedGrittyBrick That assumes level flight.  If you descend at the same time, you can go stay at 1G at that bank angle and beyond.

This whole conversation-- the question and all the answers-- seems a bit strange to me.  I guess I'm biased because in a light plane, the normal cruise speed is usually well above the best L/D speed, but my conception of the fastest way to accomplish a 180-degree turn starting from cruise speed involves pitching up into a climb to decrease the airspeed while rolling into a steep bank.  At some point before the turn is complete the airspeed will drop to the point where the flight path needs to start to descending to maintain a reasonable airspeed.

The maneuver would likely best be accomplished at full power, which will accentuate the vertical aspect of the maneuver, so that it is sort of like something like a cross between a chandelle and a wingover.  There's another ASE question in here somewhere, but I'm not quite sure what it is yet...

@Lnafziger -- re your (highly upvoted) comment "If you descend at the same time, you can go stay at 1G at that bank angle (66.5 deg) and beyond. " -- see the table in this answer to a related question -- https://aviation.stackexchange.com/a/99765/34686 -- you can see that the descent angle of the flight path has to be well over 45 degrees to reduce the G-load (lift force) to 1G in a 60-degree-bank turn,

The descent angle helps to reduce the lift that the wing needs to provide, in two ways. It leads the aircraft into higher density air, where more absolute lift is possible at the same Mach number. That takes a while, but a more immediate effect comes from pointing the aircraft's nose downwards. Then, part of the gravity-induced forces can be counteracted by drag (D in the sketch below). This is similar to a glider, only with much lower aerodynamic efficiency and steeper angles. Terry reduces thrust and opens the speed brakes to avoid overspeeding, so he can dive as steeply as possible, thus reducing lift requirements.

Now more of the wing's lift L can be employed for turning the aircraft around. If he wants to achieve a constant sink rate, he still has to produce enough lift to balance weight, but reduced by the cosine of the glide path compared to the level flight condition. At 25° nose down, this is a reduction by 10%.
Now you are concerned about his roll angle. Please note that he wants to create as much sideways component of lift as possible, which will look like gravity to all onboard. At 60° roll angle (I think his 80° are a bit extreme), this works out to twice the normal vertical acceleration, and with the dive actually just 1.8 g, so everyone will feel almost twice the force of normal gravity. With the 80° roll, this will not be a stationary turn and not enough lift will remain to counteract gravity, so the aircraft will accelerate downwards during the turn. It will actually fall out of the sky. When already at Mach 0.85, I would be more careful than Terry, because things get ugly very quickly when Mach goes further up.

The term $m\cdot\frac{v^2}{R}$ is the centrifugal force which can be afforded by the horizontal component of the lift and which needs to be as big as possible for tight turns (a smaller radius R helps). This is the lift component that Terry needs to pull the aircraft around.
If we stay with stationary turns (without the "falling out of the sky" part), the maximum bank angle is given by the maximum load factor of the 747. At 400 KEAS, this is just 1.5g, so even with the 25° nose down attitude the maximum bank angle would be 53°. If you fly slightly slower, the load factor goes to 2g (equals 63° in a 25° descent) and tops out at 2.5g at 310 KEAS (68.7° in a 25° descent). Flying at Mach 0.85 in 30.000 ft will produce an equivalent airspeed of 306 kt in normal atmospheric conditions. The precise value varies with the actual flight speed, and the maximum safe bank angle is in the region of 60° to 70°.
I expanded my answer to the original question, and I hope in combination with this I can cover your questions. If not, please keep asking!

EDIT: I see, now you want to know the time to turn around. All I can do is calculate it under the assumption that enough lift is available, regardless of buffeting. In reality, I doubt that even the 48° case will give you a pleasant ride. All results are for the 30.000 ft case, because at 36.000 ft those higher bank angles will not be cleanly flyable due to compressibility effects.
At 60° the airplane flies with 2g, and with 25° nose down still at 1.8g. The turn rate would be 3.77°/s and 180° would be completed after 48 s.
At 70° it flies with almost 3g, and with 25° nose down at 2.65 g which is a little outside of the g limits. The turn rate would be 5.98°/s and 180° would be completed after 30 s (plus the time to roll to 70°, which now starts to become important).
The 80° case becomes really hypothetical, because even with the 25° nose down attitude the load factor will be 5.2g if flown without sideslip and enough lift to prevent overspeeding. The 747 will be close to breaking apart, but maybe a future aerobatic version can fly this maneuver safely. It will result in a turn rate of 12.35° and need 14.6 s for a 180° turn. I would add at least 4 seconds to bank to 80°, but the turn can be completed within 20 s. Hypothetically.
What Terry hinted to when he said he would bank to 80° is not the clean turns I am calculating here, but a dynamic dive where the plane accelerates because not enough lift is left to keep it from falling. To compute this, I would need more aerodynamic data on the 747 and need to employ at least a finite difference algorithm or a lot of paper and pencils.
Just that much: If the 747 is rolled to 80° but the pilot pulls only as many gs as the aircraft can sustain (due to buffeting I doubt that even 2.5 g are possible), the lift will almost exclusively serve to fly a tight turn, but not enough will be left to counteract gravity. Therefore, the aircraft will slip sideways, and directional stability will turn the nose downwards. This will take a while (I am guessing maybe 12 seconds, really a guess), but the aircraft will pick up speed from the beginning of the maneuver. I am a little more scared of flying faster than Mach 0.85 than Terry is, and the various reports of 747s going up to Mach 0.98 support his view that a little overspeeding is OK. It is hard for me to guess how much time it needs to become uncomfortably fast, and then a lot of lift will be needed to pull out of the dive. On the other hand, when the airplane is diving, all it needs is a roll and a pull-out to come out at 180° to the previous course. The pull-out will be performed at a much lower altitude, and creating the gs there will not be a problem.

You say that at 5.2 g "the 747 will certainly break apart" but that's how much China 006 pulled. True, it did partially break apart, but it remained flyable. How is this different?

@raptortech97: The maximum design load factor of the 747 is 3.8, the safety factor is 1.5. If it is designed properly, the plane should break at 5.7 if everything is working as designed. The safety factor is there for a reason, and I exaggerated a little. I edited the section.

Some information in additional to the excellent answers already given:
First, let's say the need for the emergency turn occurred just after leveling from a typical step climb of 2,000 ft, which would probably mean you had only about 1.3g buffet limit protection, although in good conditions we often went up when we had only 1.2g. Given Murphy's law, let's say we're at 1.2.
Now the need for the turn comes. For the first several seconds, we dare not exceed 1.2g lest we enter a region of possible uncontrollability. We need to lose at least a couple of thousand feet as quickly as possible to get down to where we can start asking more of the wing. At the same time we want to use what the wing can safely provide to turn us. The steeper the turn the more that will be and the faster we will fall. Both good for a very short while. My seat of the pants guess is that we would roll to whatever extreme we were comfortable with, and then almost immediately start lessening the bank at a rate and to a degree dictated by what our speed was and how fast it was increasing.
The overspeed clacker should go off around mach 0.92, although in maintenance test flights I've seen it not start until almost 0.94. At whatever point it goes off, we'd want to start reducing the bank, and we'd want to replace the lost turning force and addressing overspeed by increasing the angle of attack to what the wing could safely do buffet-wise. By the time we'd dropped, say, 4000 ft, buffet speed would no longer be a factor, and we could safely pull something like 1.8g (or higher if necessary).
It would be a matter of playing our bank and angle of attack to get what we want.
In thinking through this scenario, I'm informed (or at least I think I am) by two things.
First is the incident on Dec 12, 1991 described in this NTSB report (PDF) and in this article. I was a captain at Evergreen at the time (in fact had flown that very airplane a few weeks earlier and had noticed the autopilot anomaly which was to later cause the incident), and I knew and talked to the aircrew (as did everyone at the company).
Pertinent to this discussion is the fact that the aircrew did not notice what was happening until the INS gyros tumbled (old Carousel system with actual gyros), which happened as the airplane reached 90 degrees bank as I remember. Max bank angle prior to the start of recovery was 95 degrees. Also, the engines were at cruise power until the start of recovery. There's controversy as to the max speed reached; estimates ranged from mach 0.98 to 1.09 (a mach 1.25 report was spurious). Max nose down was 35 degrees. Altitude loss was approximately 10,000 ft.
Having read the previous answers, I'm a little less comfortable with the 80 degrees scenario, but I don't think that it would mean disaster. INS platforms would still be up. Speed I feel would be manageable given that we're going to idle thrust right at the beginning, and the speed brakes would help on that. The difference, though, between using 70 degrees and 80 degrees would probably be pretty small. The pdf referenced in the first link talks about another 747 going to 71 degrees as I remember.
Second, we had a simulator exercise in which we would sometimes exceed 60 degrees of bank, although I can't say for sure that I ever went to 80 degrees. The scenario would start with departure control saying, "You have a bomb aboard timed to go off in 5 minutes. Cleared to land any runway." The sim instructor, of course, would have put you in what he felt was as difficult but still doable situation as possible. So, what you did was power off, start throwing out all the drag you could (screw the speed limitations), and roll to whatever degree of bank would start you getting lined up. The exercise always started from a relatively low altitude, so buffet speed was not an issue.
Makes me teary-eyed thinking of fun times past.

This is a very interesting story.  As a non-pilot asking this question, could you explain why a 90 degree bank would be unnoticed by crew members?  It seems like you would all be falling onto the wall in that case, but I'm sure there must be other factors at play.

@borrrden: Lift is the main force on the aircraft and since it is perpendicular to the floor, you never feel pulled sideways (much) in an aircraft, only down to the floor or possibly up. This is best explained by an image, which can't be put in a comment. Look at Peter Kämpf's answer or ask a new questions for detailed explanation.

Actually it makes a bit more sense now.  I do indeed recall feeling the sensation of being pulled down into my seat as a passenger in a banking 747, and now that I think about it not really so much to the side.  It's just a bit boggling to think about 90 degrees O_O

@Terry, could you even extend speedbrakes when you only have 1.2g of lift available? As far as I know B747 uses partial extension of spoilers as speedbrakes and these reduce lift. So wouldn't that be against what you'd need there or even unsafe (at the beginning; later on you'd have enough lift of course)?

@JanHudec What I take the question to be is whether with speed brake deployment, I would have changed the shape of the wing sufficiently that I no longer had 1.2g protection, and that even using just 1.0g (which would at first be my target) might put me into the region of a high speed buffet. I have to admit I had not thought of it from that standpoint, and I really don't know the answer. Given that, I'd now say that I would not deploy the speed brakes until I had dropped a couple of thousand feet. Good catch!

@Notts90 Yes, although typical step climbing started at FL290 (29,000) to FL310 and continued from there in 2,000 ft intervals. 747-100s and the earlier 200s could initially only get to FL290 when taking off at max gross weight. Back then the high-altitude airway system was organized in 2,000 ft intervals on odd thousands. One thousand foot separation was not generally available. As I remember, the North Atlantic track system was the first to switch from 2,000 to 1,000 separation, but I don't remember the year.

What is the maximum safe bank angle of a 747?

That might depend on who is making the safety assessment and what the alternatives are. There might be circumstances where a pilot might feel that being the first pilot to bank a 747 at 70° would be safer than colliding with a mountain or another aircraft, or being shot down.
The FAA collect "Statistical Data for the Boeing-747-400 Aircraft in Commercial Operations". For example:

I think the FAA discard from the data any bank angles greater than 40.  My guess would be that airlines and/or pilots consider angles greater than those shown in this graph to be unsafe (at that stage of flight?)
This data seems to be related to touchdowns. Perhaps greater angles are safely used at high altitude.

JA8119, a 747 SR-100, after an explosive decompression and loss of vertical-stabilizer seems later to have briefly survived an 80° roll without the aircraft breaking up. In the accident report is this record from the flight data recorder.

This aircraft was essentially uncontrollable, experiencing phugoid cycles and dutch rolls and tragically crashed shortly thereafter. However maybe it suggests an undamaged 747 might be able to survive quite high bank angles (assuming RLL in the report means roll-angle). As Peter Kämpf noted in a comment, it may not be safe to draw conclusions about the flight characteristics of undamaged 747s from this data.

Excellent data you found there! So it seems possible at least!

I believe this is actually the air disaster I was talking about btw.

The 80° bank needs not be a coordinated maneuver in level flight. If the pilot pulls up and then rolls while  pushing the nose down, little vertical lift component is needed, and 80° roll is possible with a smaller load factor than in level flight. Since the aircraft in question had lost most of its vertical tail, flight dynamics are hard to compare with a regular 747.

@Peter: thanks, I've edited the question to make clear JA8119 was a severely damaged aircraft at the time.

Normal operation, the 747 does not exceed 25 degrees angle of bank.
Steeper banks are done generally only in the simulator and will be up to 45 degrees of bank.
If radius of turn is to be decreased, then decrease speed.  Depending on weight, altitude, and temperature, this may or may not be possible.  Angle of bank that can be achieved will vary with altitude.  At 10,000', a 45 degree bank can be done; at 41,000 this would be unwise.
10 degrees nose down is excessive in the 747.  25 degrees nose down is ridiculous.  Especially while turning.
Speed is the key to reducing radius; there's only so much bank one will be able to achieve in the 747.  It's not like flying a Cessna.  If a reduction in speed is necessary per the original post, to achieve a minimum radius turn, a climbing turn would be in order, rather than a descending turn.
70+ degree turns are not going to be happening in the 747.
If the desire is to turn away from a political boundary from a given heading, a 360 degree turn will not be productive.


text:
Should the approach speed and wind be changed below 700ft on an A320?
The FCOM says:

When managed, the speed target is computed by the FMGS and may be
modified by the flight crew through the MCDU. At 700 ft RA, the
current speed target value is memorized by the autothrust, to ensure
stabilized speed guidance, even if Flight Management fails. Below
700 ft, any new VAPP or WIND entry in the MCDU has no effect on the
speed target.

When selected, the autothrust always targets the
speed selected on the FCU

I saw some captains changes VAPP and wind in MCDU below 700ft. Is this wrong?

Is the airline SOP explicitly prohibiting it? The FCOM quote doesn't prohibit manual changes, it only explains that the FMGS doesn't change the speed but manual selection on the FCU is valid.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
When carburettor heat knob is partially open, is the primary air filter closed fully or partially?
I know that when the carburetor heat knob is fully opened, the primary air filter is closed. But what happens to the primary filter when carburettor heat is partially open? Is the primary filter closed fully or partially?

Which particular aircraft are you asking about? Not all carb heats work the same way.

Use of partial carb heat is not recommended because it is possible to raise the intake air temperature to  the point that it is conducive to actually producing carb ice. Only use partial carb heat if you have a carb air temp guage.

thank you for your responses and thanks "Bianfable" 4 your assistance

The normal airbox path is through the air filter. Carb heat, as an alternate path, usually draws air from within the cowling and is unfiltered. The knob in the cockpit operates a baffle plate that opens one air inlet or the other within the carburetor air box.
You can stop it at an intermediate position, say half way, and the engine will be pulling half filtered cold air and half unfiltered heated air. At altitude, drawing unfiltered air is not too bad for the engine because there is little dust (silica particles) that can do (long term) damage. At low altitude however, you want to avoid extended operation on unfiltered air.
But in any case there is a bigger problem.
Using partial carb heat can be dangerous because you don't know the actual temperature of the blended air reaching the carb body. There needs to be enough heat to keep air at the throttle plate and venturi well above freezing, allowing for the huge temperature  drop you get (a bit) from venturi effect itself and (mostly) from evaporative cooling of the fuel.
If the airplane came with a carburetor air temperature gauge, you could limit the amount of heat to make sure the air temperature at the sensor in the carburetor throat stays several degrees above freezing and avoid too much unfiltered air. But without a temperature gauge, there's no way to know how much heat is enough, so you just apply all or nothing, even if fully on is more heat than is technically necessary at any given time.
So, due to the fact that carb heat path introduces unfilter air (undesirable) that is heated (also undesirable because it reduces performance and makes the air/fuel mixture richer), and may or may not be keeping the air going through the carb warm enough to prevent icing, use of partial heat is a bad idea all around, and can be quite dangerous unless you have a Carburetor Air Temperature gauge. So stick with fully on, and fully off. The worst time for carb icing is during descents with the throttle closed or nearly closed, so be liberal with carb heat during descents.
Fuel injected engines don't have carburetor icing issues and don't have to use an air heater, but they do need to provide a secondary air path for the engine just for safety, mostly for the case where the air filter ices over in icing conditions or heavy snow. These engines have an Alternate Air control, which operates a baffle plate in the inlet airbox similar to carb heat, but the air is unheated and is only for when the main inlet is blocked for some reason. Alternate air is, like carb heat, unfiltered air drawn from within the cowling, just not heated.

thank you for your response

Well in general, the carburetor heat knob is just opening and closing a door that controls whether the air comes in through a duct that leads to the air filter, or comes from another source that is unfiltered, but warmed by the exhaust manifold.  If the carburetor heat knob is in an intermediate position, the door is part way open and you'll get air from both sources.  Some of the intake air will be passing through the air filter, and some won't be.

¡thank you for your response


text:
What do these clearances by Heathrow Director mean?
For context: I know what a hold is, I know why we have them, but this is something I have not heard before.
I was recently flying VFR outside CAS (Controlled Airspace) with a friend of mine and we was outside the London CTA. We thought it would be interesting to eavesdrop on Heathrows' FIN Director Frequency - 120.4 on the COM 2. We both heard a clearance which we couldn't quite get our head around, it was:

BAW819K, Turn right to Lambourne, cancel the hold, leave Lambourne 265

We both look at each other and go "Eh, that sounded different". Shortly we heard it again:

EIN1LT, Make a right turn back to Bovingdon, leave Bovingdon heading 180 degrees

Could anybody explain what this means? Any explanation?

Those are both very common clearances given to aircraft that are already in the hold near Heathrow. The controller is just giving instructions on how they want the aircraft to leave the hold. It means to turn back directly to the holding point, and then leave the fix on a specific heading as their first vector.

Good evening, thank you for this answer.
I appreciate the clarification. I haven't heard of this method of clearing aircraft out of holds before. Is this documented in any phraseology document or procedure manual that I can refer to for further information? I wasn't taught this when I was doing my PPL (But was of course taught holds and hold calculations ETC) and thought some extra info would be most beneficial to me!
Thank you,
Harry


text:
Telling an unmanned aircraft from its flight path
I'm not a hardcore fan of aircraft - I just like to know what is flying overhead and why...
Some days I hear this loud annoying droning sound coming from the sky - it's usually a military unmanned craft doing strange flight paths.
Is there a way to tell if a craft is unmanned because of its flight path or sound because of the 'way' it flies the plane?
Apparently it is a Diamond DA40 NG - SkyVan or Flying Shoebox! No owner showing up.

(flightaware.com)

"No owner showing up", no but from the markings, it belongs to the fleet of a flying school,  Egnatia Aviation. You can see a touch and go at Mytilíni in this video, and there are pilots onboard ;-)

I am not sure where people are obtaining the A/C type from this, but, judging from th  Ed flight path and SX-AEN’s previous flights, it appears to be a training aircraft based out of Philippi, Greece, and the flight path indicated above, appears to be an instrument training flight, probably early to mid phase of training for an instrument rating, including holding patterns, DME arcs, and procedure turns.
If it is, in fact, a diamond DA 40, or Shorts Skyvan, it is almost certainly not an unmanned aircraft, as both of those types require a human flight crew aboard in order to operate.

Well done also explaining what is (likely) going on there

Source for aircraft type: Flightradar24

In general, you cannot distinguish manned from unmanned aircraft based on the flight path or sound. The best way to distinguish them is to identify the aircraft type, either visually or via a flight tracking website (like you have done).
The aircraft in your screenshot is a Diamond DA40:

(image source)
This is a single engine piston aircraft, which is always manned:

Minimum Flight Crew: 1 (Pilot)

(EASA Type Certificate Data Sheet for the DA40)
This type of aircraft is often used for flight training. The flight path shown in your screenshot looks like it could be a training flight. If you are sure that it's a military aircraft, then it's probably flight training for the Hellenic Air Force.


text:
How does increasing the size of a jet engine make it more fuel efficient?
The design trend is there: 8, 4, 3, 2 jet engines on powering the large cargo or passenger aircraft.  Is it mostly to accommodate the larger, more efficient fans, or are there also benefits in the combustion cycle?
Jets seem more immune to the issues of excess heat removal that their piston engine predecessors suffered when scaled to greater size.  Is there a limit to how large an axial jet engine can be?  Are we approaching that limit today with engines such as the GE9X at over $130 \, 000 \, \text{lbs}$ of thrust, or is there more to go?

One aspect of this is bypass ratio. Increasing the bypass ratio increases fuel efficiency (and noise reduction) and that increases the overall diameter of the engine. So it's not so much the engine core is getting bigger as it is the bypass ratios are increasing.

What cargo/passenger aircraft has 8 engines? There are a few 3-engine MD-11's and MD-10/DC-10's in cargo operations. The trend though is moving to 2 engine aircraft, like the 777, 787 and A340/350, most "large" aircraft with 4 engines are going away, like the 747 or the A380. Are you saying that the trend is moving to larger but lower quantity of engines? Maybe I misunderstood your first sentence.

@RonBeyer: In that particular case the cargo of the 8 engined jet is bombs.

@RonBeyer https://en.wikipedia.org/wiki/Category:Eight-engined_jet_aircraft

@nick012000 I know of 8-engined military aircraft, there are none that I know of that are cargo/passenger ones... And in the article you linked, only one of those aircraft went beyond the concept stage, the aircraft that Peter Kampf eluded to, the B-52, otherwise there are none.

Related: is it currently possible to build a jet engine with 1050 kN of thrust

@Ron Beyer it was/is the B-52. This 65+ year old masterpiece has an MTOW similar to a 787-8.  No, the 787-8 does not have 8 engines.

It isn't a cargo aircraft though, it's a strategic bomber, it doesn't carry anything other than bombs. It also had a bad habit of the tail breaking off in flight.

GE also has some fairly huge gas turbine electricity generators.

Related: What prevents jet engine designs with larger bypass ratios?

Should be noted that the choice of 4 vs 3 vs 2 engines often comes down to legal operational and design rules. Even modern 777/787 routes are limited to a certain distance from planned emergency landing strips, designated in minutes of flight at one engine speeds. They are also more limited in terms of minimum allowed takeoff weather, as well as design performance with one engine failure. Twins need more oversized engines because they loose 1/2 of max thrust while quads only loose 1/4, also twins have major engine out handling changes with all thrust being on one side, so bigger tails, etc.

8 on the B-52 was really more a matter of using what was available at the time. Also beneficial to use one engine design on several planes for maintenance reasons. And being military they may consider the effects of engine damage on the mission and engine size as target size. B-52 was built with carrying live nukes for the strategic air command as a primary mission, so redundancy likely had a heavy weight in the calculation.

The efficiency issues are as follows.
For a fixed amount of mass flow rate through a work-extracting thermodynamic process, one large engine is more efficient than two smaller ones because the pair of smaller engines present more interior surface area relative to the mass flow through them than the single larger engine. The more wetted area inside the engine relative to the mass flow, the greater will be the frictional losses. This drives the designer toward fewer and larger engines and away from more numerous and smaller engines.
In addition, there are aerodynamic losses associated with each engine nacelle/pylon assembly on the wing which are minimized by reducing the number of engines installed per wing.

+1. Add to that that internal tolerances will become smaller relative to overall size with larger engines. This reduces gap losses. The biggest gain, however, is from volume to surface ratio and increased Reynolds numbers, as you correctly explained.

Larger engines don't need more blades? How can blades further appart do the same work as closely packed blades?

Is there an upper limit to the blade length due to transonic radial velocity of the outter radius of the blade (similar to wind turbines)?

there is an upper limit to the blade length set by 1) centrifugal stresses in the blade material and 2) the need to keep the blade tips subsonic.

A larger jet engine is more fuel efficient due to there being fewer losses from:

Smaller boundary layer effect.
Lower frictional bearing losses.
Lower relative tip loss effect.

Boundary layer. Gas flow through a stream tube experiences energy loss at the tube wall: in the boundary layer flow velocity is reduced due to viscosity. This causes an effective pressure loss in the gas stream, and the efficiency of a Brayton cycle such as in a gas turbine is mainly a function of the pressure ration.
Frictional bearing losses. The tip speed of both turbines and compressors are limited by compressibility effects. A linear speed limit translates to a lower rotational velocity when longer blades are involved, which implies lower friction losses in the rotational bearings.
Tip loss. At the compressor and turbine blade tips, there is an end loss effect similar to that at wing and rotor blade tips. Inside the stream tube of a gas turbine engine, the tips have a small gap between blade end and tube wall, the smaller the lower the effective losses.

The three loss factors have a certain absolute magnitude - proportional to the gas flow volume, they become smaller as the gas flow volume becomes larger. And lower loss means higher fuel efficiency.

The square-cube law favors large heat engines, because losses are lower the bigger the engine is, since, in general terms, losses are related to surface, and output is related to volume.

Other answers have well explained the real-world implications of engine size and number. Here's a brief take on why an idealized larger-diameter engine (or propeller) is in general more efficient than a smaller one to produce the same thrust. Skip to the end if you're not concerned with the mathematical details.
Consider the following situation (source):

The fan accelerates the flow and the streamtube is narrowed. Depending on the type of engine, this narrowing happens in free air or within a nacelle. Let's say the flow has a velocity $V$ at point $0$ and a velocity $V+v$ at point $3$.
The propulsive efficiency $\eta$ is the ratio of the useful thrust produced by the engine and the rate of work expended to achieve this thrust. The useful thrust is given by $\dot{m}v$, where $\dot{m}$ is the mass flow through the engine. By the work-energy theorem, the rate of work expended is equal to the rate of change of kinetic energy, which is given by $\frac{1}{2}\dot{m}\big((V+v)^2-V^2\big)\implies\dot{m}v\big(V+\frac{v}{2}\big)$. Therefore,
\begin{equation}
\eta=\frac{\dot{m}v}{\dot{m}v\big(V+\frac{v}{2}\big)}=\frac{1}{1+\frac{v}{2V}}\,.
\end{equation}
So to drive the efficiency towards its maximum at $1$, we want $v$ to be as small as possible. That is, the most efficient engine imparts the least velocity to the airflow. But remember that the useful thrust is $\dot{m}v$, so if we decrease the imparted velocity but want the same thrust, we must increase the amount of air flowing through the engine. The easiest way to do that is to simply make the fan bigger.

Other people provided very detailed responses that certainly describe many aspects of large engines, but I think the key variable here which have the most impact is the bypass ratio. Turbofan engines generate large proportion of thrust from the air that passes through the fan, but bypasses the engine.
Technological advances in the (core) jet engines probably enabled this trend by increasing the power on the shaft without corresponding increase in weight. The best way to employ this additional power under constraints of passenger travel is to increase mass of air that passes the fan, but not the engine - the bypass ratio.
This cannot be done by increasing the speed of the fan, as you start hitting the material limits on large diameter fan blades. So geared turbofan designs started to appear, which allow large diameter, slowly rotating fan, which provides thrust, and high RPM core, operating at maximum efficiency.
This also has a side effect of reducing noise.

I will attempt to answer your question without any equations.
What defines propulsion force of jet engine? Mass and speed of 'ejected' gasses and also surface the gasses will pushed against (atmosphere), minus air resistance created by engine on intake side (air sucked in but still engine creates some resistance).
Theoretically we can push gasses faster by narrowing gas eject opening, but at some point we will reach critical velocity when gasses will have not enough surface to push against.
Also we can not make intake opening too big -- resistance will start overtake efficiency at some limit value.
Also we should take in account a mass of the jet engine -- more weight means more load on engine/wings and efficiency decreases (more fuel burnt to carry the engine itself).
Force generated by jet engine is proportional to square speed of 'ejected gasses' multiplied by mass of 'gasses' itself [I=V^2*M].
A balance should be achieved to make jet engine efficient.
Two jet engines is less efficient than one due equation S=Pi*R^2 -- surface increases faster with increase of radius.
Two or more engines increase reliability of flying craft -- but at expense of efficiency. Nowadays airplane can land with only one engine, it was put into design of air craft.


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What is the main source of yaw stability?
I am creating a flight simulator and I need to implement lateral and longitudinal stability.
Roll stability is now working because I am computing the lift of each wing independently.
But yaw stability is confusing to me.
What is the main source of yaw stability : is it the weathervaning effect of the tail or is it the sweep angle of the wing ? Which one is the more responsible of stability ?
When I implement weathervaning effect by applying the wind at a certain point of the tail, the problem is that the oscillation will continue and the plane will always overshoot as he is not losing any of its angular velocity. So it will never align into the wind but will keep overshooting from negative to positive angle.
So my other question is : in real life, what does prevent the plane to overshoot wind direction?
When I just apply some scaled-angular drag value to the plane, it helps to dampen the oscillations, but the plane is wayyy to slow to align into the wind and thats just aweful to controll and not realistic at all. So I guess angular drag is not the answer why oscillations stops in real life. There must be another force depending on wind speed or I don't know...
And longitudinal stability is also confusing to me.
I know that it depends on the position of CG, CL and the downforce generated by the tail horizontal stabilizer.
But what about the weathervaning effect ?? Isn't it the main force that explains why the plane align into the wind on its pitch axis ?
As you can see I am a bit confused. I already looked many websites and I mainly understood how stability and flight dynamics work, but as I am working on a simulation, I need some equations to implement, and some concrete values to understand which forces are bigger than others.
Thank you !

One thing to consider is that oscillations are an issue in real aircraft and many have yaw-dampers specifically designed to actively combat this situation. Depending on the aircraft you're modeling, you may not get a purely aerodynamic solution to dampening and the plane may continue to oscillate no matter how you model it.

Thanks. But as I said, I am making a flight simulator and the plane I am modeling is a Pitts Special. As an aerobatic plane I am waiting for it to stop oscillating, so I am searching for a way to implement this

Roll and yaw are heavily coupled in conventional aircraft, it makes little sense to discuss roll and yaw stability as separate phenomena. The longitudinal modes can be resolved separately, but roll and yaw share modes. For more information, see here: https://courses.cit.cornell.edu/mae5070/DynamicStability.pdf

Maybe it makes a little sense to discuss them at separate phenomena, but as I said, I am trying to implement a COMPUTER SIMULATION. That means I write code, so I need concrete forces and torque to apply, so I am not searching for some explainations of how we can represent the damper phenomena, I am trying to return to the roots and apply the forces that CAUSES them.
We can simplify the problem, and I would like to simply know what causes a weathercock to not overshoot wind direction, and why it dampers pretty quick into wind direction.
If I can modelize that I'll be good.

@Anselme the source I linked has a thorough discussion fo the physics involved, your model should end up looking like that. Implementing it on a computer does not make much of a difference, if anything, it adds complexity due to the discretization, so I suggest you look at the analytical approach first.

Wing sweep does affect yaw stability, but all planes with a vertical tail will "weathervane" into the wind.  It seems you are on the right track considering angular drag as a mechanism of stability, angular lift works even better.  A vertical stabilizer can act as a vertical wing.  Symmetrically, it can generate lift against either deflection.
Secondly, the "overshoot" is a real concern in design, but will be a function of weight as well as "angular drag". Much harder to stop a piano on ice compared to a light cardboard box.  A tail will typically be light, pivoting around the center of gravity.  Ample tail area will make it easier to control.
Finally, as far as "damping" is concerned, one must realize the "viscosity" of air will resist motion as well.  Once a force is removed, drag will help it slow to a stop, so the opposite "correcting force" need not be as large.

Thank you very much.
The problem is when I compute angular drag, it's always the same at high speed and low speed. If I want the tail to align quick into the wind without too much oscillations, then it forbids any quicker rotations even on 0 airpseed, not allowing me to do Lomcevak for examples.
That's why angular drag seems not realistic at all and it does not seem to be the way to achieve it.
Maybe considering lift could do the trick, I will try it.
Yeah, of course overshooting depends on weight but that does not tell me how to compute a realistic weathervane effect

Here is how I want it to work : get the tail align quick into the wind without too many oscillations, like on a real aircraft, but allow for some realistic aerobatics like Lomcevak for example. Any angular drag value high enough to dampen the yaw oscillations after a perturbation are completely killing the momentum of the plane on its yaw.
I don't know if I am clear enough

@Anselme I'm seeing using lift (from rudder deflection) yawing the tail.  "Angular drag"  will stop rotation at a certain deflection point. Angular resistance to the yawing force is 0 at first, increasing as more tail area "comes into the wind".  So, angular resistance increases and yaw "lift" decreases as tail "weathervanes". Even with rudder still deflected, yaw and resistance to yaw forces reach zero, and yaw stops.

The only other consideration is the resistance to movement of the air itself.  I would make sure I'm not modelling movement through water, but air "viscosity" will help dampen any movement quickly once forces balance to zero.  The "flutter" case should only occur if the tail assembly is elastic enough to bend and store energy as a spring.

Thank you.
I don't know if what I am doing is right, but I did lilke "quiet flyer" told me to do.
See the answer below
My simulation works quite well now and the yaw dampens well.
What I am doing is computing the lift of the vertical tail, AND the lift of the fuselage side. As they are not on the same 3D point they don't have the same AOA, thus it creates a counter-yaw that helps dampen.

Rolling is also one of the main source of yawing moment. Because when aircraft is rolling, lift of the wings is not equal to each other. This causes different induced drags for each wing. Drag difference of the wings generates yawing moment. You need to consider that one also.

Do you have any sources to back up your claims?

It is not a claim. It is pure aerodynamics. You can refer any aerodynamics text book or faa flight manuels for beginner pilots.

Thank you for your answer, yeah I am already taking into account this effect as I said, each wing lift is computed separatly. But that does not answer my question

There is an aerodynamic damping effect in yaw just as there is in roll.  Imagine that the whole aircraft has somehow become yawed 10 degrees to the flight path, and then whatever torque has caused this to happen has vanished.  The fin will generate a yaw torque that makes the plane start to yaw back toward the flight path, but as it yaws, the rotational velocity changes the local linear velocity of different points on the fuselage, which changes in the "sideways angle-of-attack" of the forward parts of the fuselage in a way that decreases the net yaw torque on the aircraft.  Specifically, the yaw rotation will increase the "sideways angle-of-attack" of the forward parts of the aircraft and decrease the "sideways angle-of-attack" of the aft parts of the aircraft, including the vertical fin.  At some high enough rotation rate, the net yaw torque would be zero even though the fin is still "feeling" a sideways component in the airflow.
One way to envision is this is to understand that in steady turning flight, whenever the aircraft's yaw and pitch rotation rates are stable and fully synchronized to the turn rate, we can envision the "relative wind" or free-stream airflow to actually be curved to follow the path of the turn, so that different points on the fuselage cannot all have zero "sideways" angle-of-attack, nor can they all have the identical pitch-wise angle-of-attack.
In short, the rotation of the aircraft in the yaw axis causes different parts of the aircraft to be moving through the airmass in different directions at any given instant in time, so that "yaw strings" attached to different points on the aircraft would be angled in slightly different directions.  To read about how one pilot takes this into account in actual practice, see the Soaring magazine article Circling the Holghaus way by Richard H. Johnson.  Links to some other articles discussing the curvature of the relative wind in turning flight are embedded in this ASE answer.
As for your question about the "weathervane effect" in the pitch axis, you could say that the balancing act between the wing and the horizontal tail is sort of like a "weathervane effect".  Airfoils generally generate a nose-down pitching moment.  Since apparent "center of lift" of the wing (including the effect of this nose-down pitching moment) is usually not at the CG of the aircraft, the point where everything comes into balance will usually not be the point where the horizontal tail is generating exactly zero lift.
Naturally, there is an aerodynamic damping effect for pitch rotations as well.  The difference in angle-of-attack between the wing and the tail required to command a given angle-of-attack of the wing in turning flight will be different than in linear flight, especially at steep bank angles where the turn mostly involves pitch rotation.  Again this is due to the fact the pitch rotation creates a local change in angle-of-attack that varies from one point along the fuselage to another.  Generally we have to have the stick further aft to command a given angle-of-attack of the wing in turning flight than in linear flight. This is effect is especially pronounced in slow-flying aircraft like sailplanes.  If the fuselage could "bend like a banana" to match the curving relative wind in the turn, this effect would vanish.
The idea of a "curvature" in the free-stream "relative wind" is really just another way to look at aerodynamic damping due to rotation about the pitch or yaw axes, while aerodynamic damping due to rotation about the roll axis can be viewed as a being due to a "twist" in the free-stream "relative wind".

Thank you again.
"There is an aerodynamic damping effect in yaw just as there is in roll."
I managed to get this roll damping effect by computing and applying lift on each wing independently. And that worked perfectly showing the simulation is accurate.
However I am not understanding your explaination. Or maybe I did...
Are you saying that when I am yawing RIGHT, the right wing looses speed, so looses lift, so looses drag, so get quicker, while its the opposite onthe LEFT wing ? Is that supposed to dampen my yaw perturbations ?

I am thinking of an effect coming from the fuselage itself, but the change in drag from each wing might also play a role.

I am not sure to understand why those different yaw directions on different points of the fuselage would help damper the yaw moment... I am reading the link you gave me but still...

Added fourth sentence in first paragraph, may help.


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At which point does an airliner start to pressurize/depressurize the cabin?
Do airliners start changing cabin pressure at takeoff or after reaching a certain altitude? Also when do they start depressurizing the cabin ?

see also this answer: https://aviation.stackexchange.com/a/16799/1467

This diagram shows (dotted line) how "cabin altitude" is managed during the flight.

The cabin pressure is maintained automatically by a controller. Before the flight, the pilots will input two numbers: the cruise altitude and destination airport altitude. From these two inputs, the controller will manage the cabin pressure schedule automatically.
From the Boeing 737 FCOM:

The cabin begins to pressurize on the ground at higher power settings.
  (...)
In the air, the auto controller maintains a proportional pressure
  differential between airplane and cabin altitude. (...)
The descent mode is activated when the airplane descends 0.25 psi
  below the selected FLT ALT. The cabin begins a proportional descent to
  slightly below the selected LAND ALT. (...)
While taxiing in, the controller drives the outflow valve slowly to
  the full open position depressurizing the cabin.

For passenger comfort, the controller is programmed so that the rate of cabin pressure change is kept to a minimum. The cabin is actually slightly pressurized on the ground both during takeoff and landing, i.e. the cabin altitude is lower can the outside altitude. It smoothly transitions to and from cruise cabin pressure as the plane climbs or descends.


text:
What would happen if you shot an F-16's fuel tank?
Let's suppose you have a 20mm sniper rifle and shoot the internal fuel tank of an F-16 coming directly towards you. What would happen ?

A 20mm sniper rifle?  Well, you would need your shoulder reconstructed, if the F-16 flying into you didn't cause you significantly more damage than that.  And there would be a hole in the fuel tank.

Yes they exist Anzio 20mm and you can simply mount them on the balcony rails and connect stock of it to an hook on the roof (But it will hard again since mounting them doesnt eliminate shoulder pain...)

... what is this supposed to accomplish? self sealing fuel tanks and generally godly damage control by the F-16's computers eliminates any critical damage caused by ripping through control surfaces. Just as @TomMcW says, if you were such a bad shot that you missed the fuel tanks and hit the pilot, I'm pretty sure you'd be the unluckiest and at the same time stupidest 'sniper' in the world. And you'd destroy the aircraft.

Depends on where it hit the airplane and what other systems were subsequently damaged.  A typical rifle bullet will cut through the aluminum structure of an aircraft like a knife through butter.  If it was a clean shot where the bullet penetrated the tank and exited the aircraft, hitting nothing else, it would result in minor damage and a small fuel leak, quickly sealed by the tank liner.  If the round hit major structural members on its path, this could structurally compromise the aircraft.  Most dangerous would be a hit where the bullet penetrated the tank, then exited and pierced the hot section of the engine.  This would allow hot exhaust gases to escape the engine and could easily cut into the fuel tanks like a blowtorch, causing internal fires and possibly the loss of the airframe.

I'd say most dangerous would be if the bullet penetrated the pilot

@TomMcW That thing REALLY happened, when Japanese attacked an another aircraft carrier, an US pilot ejected from his seat and shot the Japanese pilot Owen J. Baggett
And something like that will never happen again (Unless you are playing BF3 https://www.youtube.com/watch?v=LrOIgxQ--Tc )

It’s happened.  The majority of kills from WWI are a result of that.


text:
Who made the first high-wing jet airliner?
I thought the only high-wing jet airliners were made by the Soviets but the British Aerospace RJ-100 and Dash 6 series twizzled my mind.
So who made the first high-wing jet airliner?

Perhaps the Baade 152, built in Germany (GDR) in the late 1950s...
https://en.wikipedia.org/wiki/Baade_152

Good find.  But if the criteria is airplanes that actually made into production, I'd say the 146 is it, at least in the West.


text:
Why aren't cockpits airtight?
There are accidents where the pilots couldn't see the instruments and literally flew the aircraft blindly due to the excessive smoke in the cockpit, ranging from UPS Flight 6 to other accidents.
How come no company has made an airtight cockpit to prevent smoke from entering the cockpit? Even though it can be said it is complex I think it doesn't make an multimillion airliner a lot more expensive?

What happens when the smoke originates in the cabin?

@RonBeyer a system that vacuums the air? Or better an system that vacuums the smoke to an airlock?

Ok, so what happens when there is smoke on the other side of the vacuum system and it blows smoke into the cockpit? Are we just abandoning the passengers at this point? How do we design for failure modes for all the wires, control cables, tubes, etc that need to go into and out of the cockpit? There is an easier solution that exists... Smoke hoods.

@RonBeyer the problem 1 doesn't apply to cargo craft and that's all I can say. Thanks for the answer

See How to land the aircraft when the flight deck is full of smoke? for equipment to use in case of smoke in the cockpit.

Cockpit smoke will come from either of two sources:

The air conditioning pack supplying it, such as the air cycle machine gone bad (more or less a turbo charger like device) with bearings and seals failed.  This usually results in smells more than smoke.  Another one is smoke from the engine compressor - unusual because are aren't really major sources upstream of the bleed ports unless something is introduced from outside.  Cooked deicing fluid is the biggest source.  Fluid gets in the compressor, and the temperature at the bleed discharge ports can be 500-800F at takeoff thrust.  If the bleeds are started right after deicing without a clean-out engine run first to purge the compressor, you'll get stinky haze in the cabin/cockpit from cooked fluid.  This has caused many a turn-back.  Another one would be smoke from an APU that blows up while supplying bleed.
You can also get smoke from burning out fan motors in supply ducting, but cockpit smoke so intense that it causes a control crisis is almost always from burning wiring in the cockpit itself, such as behind control panels or interior panels (see Swissair flight 111).

So the main thing is, smoke coming from the air conditioning system can be reduced by turning the system off, but that isn't the problem.   It's smoke originating from within the cockpit itself, mostly from wiring.  Considering how rare those events are, it's not really practical to create a hermetic seal between the space behind interior panels and the interior space and still have a maintainable and reasonably light airplane.

Which Swiss Air accident?

The one that burned up and crashed off the east coast of Canada from a wiring fire in the roof from, I believe, the cabin entertainment system.

OH I remember Mayday Fire On Board

A smoke-tight seal is not all that heavy or a maintenance nightmare. But there's too much to potentially seal. The killer is the need for a separate ventilation system for the cockpit. Generally systems are designed to keep smoke from getting into the cockpit, but that design often doesn't stand up to real-life fires.


text:
What is this military turboprop aircraft?
I took a video of this military turboprop aircraft on August 6, 2020 at 7:03 PM in Long Beach, California. Here is a screenshot of it:

You should invest in a telescopic lens.

From that really grainy, blurry picture, it looks like a C130 Hercules.

bulbous fuselage
Straight, ironing board-like wings
High wing configuration
4 turboprop engines
Huge, high empennage
The rear is shaped like it could have a ramp.

If you are sure it was a turboprop, it looks like the C130 is a strong contender.

I agree that’s about the only thing it could be.  The C-130 doesn’t have a T-tail though.

@MichaelHall - My mistake. I must have been getting it mixed up with the turbofan C17 Globmaster.

It looks like a C-130 tail to me. You just can’t see the vertical stabilizer from below the horizontal stabilizer.

@MikeSowsun - It does look like the C130 tail. I remembered incorrectly whether the horizontal stabilizer was mounted directly on the tail or on the top of the vertical stabilizer. I spent time jumping out of both the 130 and the 17 too long ago to remember all of the details.


text:
How large are the CF-100 Canuck wingtip fuel tanks?
How big are the CF-100 Canuck's wingtip fuel tanks and what is the maximum fuel capacity they can carry?

This diagram from the RCAF Canuck Aircraft Operating Instructions states the wingtip tanks could each hold up to 292 imperial gallons or 350 US gallons.


text:
What's the difference between the "100 above" and "100" GPWS above callout?
Not all aircraft have both; this one does and can be heard at 1:12:

"100 above" refers to being 100 ft above the decision height, which is the height at which the pilot must initiate a missed approach if they do not have the required visual reference. The equivalent call on a Boeing is ‘approaching minimums’. The decision height or minimums will commonly be around 200 or 300 feet, but it is airport specific, so when you hear the "100 above" call, you are 100 ft above 200 or 300 ft (300 ft or 400 ft above the ground respectively) - or whatever the decision height is for your specific approach.
The "100" call means that you are 100 ft above the ground.
The audio clip in your video is a bit confusing, and probably not taken from a real flight. During a real approach, you would expect to first hear "100 above" followed by "minimums", since "minimums" is the call made at the decision height.

The audio clip in the video is a ground test of the GPWS and TCAS systems, which runs through all the calls.

Above ground or above runway elevation? That is, will you hear the “one hundred” call in the typical place when you are approaching a table-top runway with more than 100 ft drop closer than 1000 ft from the threshold?

@JanHudec Good question. For Boeing the 1000 and 500 callouts are based on barometric altitude above runway elevation and the rest is RA (see this answer). If I read the A320 FCOM correctly, they are all based on RA.

@JanHudec Probably depends on whether you have inserted a decision height (DH) or decision altitude (DA)


text:
HEPA filters, airliner HVAC and COVID 19
There is this report that says the risk of aerosol spreading on a commercial liner is low due to HEPA filters.
But it is not considering the cases when A/C is OFF. This seems to be quite common when the plane is on the tarmac but away from the gates. At the gate the air is supplied by the airport. There are numerous reports of planes being delayed for hours in hot weather and passengers suffering. I have been in that situation myself a few times.
This is explained on quora:

It's kind of a double-edged sword. Simply put, cabin air in flight comes from intakes just behind the engine compressors. In flight, you're getting fresh, clean air that is inducted, mixed and filtered. It circulates through the cabin and eventually, lots of the circulated air gets sucked out by air blowing past the plane at nearly the speed of sound. However, its a different story when you're sitting on the tarmac. Engines at idle are still producing fumes, and these can get inducted along with fresh air because there's no air thrusting the fumes away from the plane as there is when you're moving at 500 knots. That mix can be filtered to a degree and is safe to breathe, but it's still going to smell awful, and even burn your throat. If you fly often, you'll often notice an awful smell of fuel when you get on the plane that clears up after takeoff. Granted, some of that smell at idle also comes from open passenger doors, but you get the idea. Also, due to changes resulting from in-flight fires, and 9/11, the pilots are isolated, and can often be unaware of how uncomfortable it is getting in the passenger cabin. Someone has got to bring it to their attention. Its not like a thermostat in your house, where you just set it and forget it. The crew has to control the cabin air on a more or less manual basis. Ever been to a restaurant that's freezing cold? Lots of times, the thermostat is controlled by people working in the hot kitchen, and the kitchen crew may not be aware (or even care) that their comfort is turning your fingers into icicles. Same principle.

So if the AC is OFF on the tarmac due to fumes, would it not make sense to use 100% recirculation in this instance and switch to fresh air whilst airborne? Can the pilot control this from the cockpit?

Quora, the unimpeachable source of knowledge and wisdom.

Recirculated air without packs will get quite warm very quickly... terrible result. Using engine or APU bleed to run the packs is the right answer. The quote's concerns about "fumes" aren't entirely off-base, but they are significantly overstated.

The air that enters the cabin of an airliner usually comes from a mixing manifold, like e.g. shown here for the Boeing 737:

(Boeing 737 NG FCOMv2 2.31.7 Air Systems - Air Conditioning System Description)
There are three ways to supply air to the mixing manifold:

From the PACKs (Pneumatic Air Cycle Kits): these are the air conditioners on the aircraft, which are supplied by hot and high pressure bleed air (either from the engines, the APU (Auxiliary Power Unit) or via an ASU (Air Start Unit) on the ground). During flight, the engines supply the bleed air to the PACKs, which results in fresh air entering the cabin.
From the Ground PCA (PreConditioned Air) hose: here the air conditioner is on the ground (usually in the airport building) and it supplies conditioned air to the mixing manifold while the PACKs are still off. See also What is this tube connected to a 757 for?
From the Recirculation Fan: here air is taken from the cabin and the fan returns it into the mixing manifold. You can see the filter in the diagram in this related answer in front of the recirculation fan, which is the HEPA filter preventing the spread of a virus via the mixing manifold. Even in flight, some air is always recirculated to reduce the required amount of bleed air from the engines.

Your question seems to focus on the time where the aircraft has already left the gate (so ground PCA is out of the question), but is still on the ground. At this time, the engines are usually running, so they could supply fresh bleed air to the PACKs. For longer waiting times, the engines are usually switched off to save fuel, but the APU would be running to power the aircraft. The APU could then also provide fresh bleed air to the PACKs.
As the quote from quora you included shows, this bleed air is not always of the best quality on the ground. It can therefore be switched off by the pilots. The following picture shows the bleed air panel of a 737-800:

(Boeing 737 NG FCOMv2 2.10.2 Air Systems - Controls and Indicators)
At the bottom are the bleed air switches for the engines and the APU. When they are switched off, the only source of air into the mixing manifold is the recirculated air. So in this case, you are already running on 100% recirculation, which is then HEPA filtered. Since the air does not go through the PACKs, it is not conditioned though, so it will get hot and stuffy in the cabin.
The top row of switches on the panel are for the two recirculation fans. They can also be switched off, in which case no air would enter the mixing manifold any more. This would make the cabin even more uncomfortable on the ground and of course also remove any HEPA filtering.

You are right, external preconditioned air provided at the gates by airports do not pass throw HEPA filters.
Many countries' Civil Aviation Authorities (CAAs) have forbidden or restricted its use based on ICAO recommendations.

Operations without the air conditioning packs or external pre-conditioned air (PCA) source should be avoided. External air sources are not processed through a high-efficiency particulate air (HEPA) filter. Use of the aircraft APU should be permitted at the gate to enable the aircraft’s air conditioning system to be operated, if equivalent filtration from PCA is not available.
Source: Air System Operations; icao.int